How can I make stationary lava with the fluid simulation in blender?
You can setup a fluid sim and start simulating some fluid, as the simulation is baking you can progress through the timeline to see each frame as it is created.

Once you get to a frame you like the look of, you can cancel the bake. Under the modifiers properties of the domain object you will find a Fluidsim modifier, by clicking apply you will remove the simulation and leave the domain object permanently shaped as you see it, as a piece of water frozen in time.